Is it possible to call a function contained in an addon (__init__.py) already installed in blender
Yes.  Actually the next version of the glTF export addon (for upcoming Blender 2.82) will contain a new ability for 3rd-party addons to place glTF extensions into the exported file.  The code for this feature looks something like this:

    if bpy.app.version &lt; (2, 80, 0):
        preferences = bpy.context.user_preferences
    else:
        preferences = bpy.context.preferences

    for addon_name in preferences.addons.keys():
        try:
            if hasattr(sys.modules[addon_name], 'glTF2ExportUserExtension') or hasattr(sys.modules[addon_name], 'glTF2ExportUserExtensions'):
                extension_panel_unregister_functors.append(sys.modules[addon_name].register_panel())
        except Exception:
            pass


In the code above, all addons are searched (via preferences.addons.keys()), but of course you could limit your search to a specifically-named addon, if desired.

Once an addon is found, hasattr is used to look for a specific class name within that addon.  It will call a register_panel function within that class, to register the custom extension's options into the glTF file export UI.